Question

I am trying to draw a simple quad with OpenGL.The same code wouldn’t fail if I weren’t using the depth and draw the quad with 0.0 as z coordinate.
It seems like the depth isn’t enabled or whatsoever:

#import <OpenGL/OpenGL.h>
#import <GLUT/GLUT.h>

int width=500, height=500, depth=500;


void init()
{
    glEnable(GL_DEPTH_TEST);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glViewport(0, width, height, 0);
    gluLookAt(250, 250, 250, 250, 250, -250, 0, 1, 0);
    gluPerspective(45, 1, 1.0, 200.0);
}


void display()
{
    glClearColor(0.0, 0.0, 0.0, 0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glColor4f(1, 0, 0, 0);
    glBegin(GL_QUADS);
    glVertex3i(100, 100, 100);
    glVertex3i(100, 300, 100);
    glVertex3i(300, 300, 100);
    glVertex3i(300, 100, 100);
    glEnd();

    glutSwapBuffers();
}


int main(int argc, char * argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowPosition(100, 100);
    glutInitWindowSize(width, height);
    glutCreateWindow("Test");
    glutDisplayFunc(display);
    init();
    glutMainLoop();
    return 0;
}

I just see a black window, but I should see a red quad inside it.

Answer 1

Change your init() function to:

void init()
{
    glEnable(GL_DEPTH_TEST);
    glViewport(0, width, height, 0);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45, 1, 1.0, 200.0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(250, 250, 250, 250, 250, -250, 0, 1, 0);
}

Your problem is that gluLookAt() should be used to define the model-view matrix, but you are multiplying the projection one. Also the glViewport() call is unrelated to the matrix calculations, so I moved it to the beggining of the function. Not really needed, but I find that clearer.